Depending on the types and manufacturers, display devices are driven by clock signals of different frequencies. Accordingly, it is desirable that display control apparatus for computer systems and the like be capable of selectively providing a plurality of clock signals of different frequencies so that the apparatus can be used with different types of display devices.
A conventional general-purpose display control apparatus comprises an oscillator block and a display control block. The oscillator block includes a plurality of normally oscillating oscillators, and the display control block has a selector for selecting only one of the oscillation signals from the oscillators that is designated by a command signal. The display control block also includes, in addition to the selector, various circuits for operating display devices.
In general, the display control block is fabricated in a single integrated circuit, but it is difficult to assemble the oscillators in the display control block. Therefore, the oscillator block is usually coupled by appropriate connector means to external connector terminals provided on the display control block. To provide a number of external connector terminals on an integrated circuit is restricted due to technical reasons and, therefore, the number of oscillators which can be coupled to the display control block is also restricted. Accordingly, it is apparent that if the number of external connector terminals which can be provided on the display control block is, for example, four, only four different clock signals can be selected.
A first object of the present invention is to provide a display control apparatus including a display control block which can selectively provide a greater number of different clock signals than a relatively small number of external connector terminals on the display control block provided for connection to an oscillator block. Thus, the oscillator block can include a greater number of oscillators than the number of the external connector terminals of the display control block.
A second object of the present invention is to provide such a display control apparatus as stated above in which power consumption and heat generation in the oscillator block are suppressed. If all of the oscillators in the oscillator block are simultaneously in an operating state, power consumed in the oscillator block will be greater as the number of the oscillators increases and the amount of heat generated also increases accordingly. This is disadvantageous when the system is to be formed compact. Suppression of power consumption and generation of heat in the oscillator block is achieved by operating the only oscillator which generates a clock signal at a selected frequency and supplying no operating current to the other oscillators.
Another object of the present invention is to provide a display control apparatus including a general-purpose display control block to which not only the above-described oscillator block including a greater number of oscillators than the number of external connector terminals of a display control block, but also a conventional oscillator block including the same number of oscillators as the external connector terminals on the display control block can be connected, and in which proper operation of the display control block can be achieved by simply switching an operating mode of the display control block.